Tells me that you don't have the exposure set properly.
In any of the still photography exposure modes the camera tries to act like a DSLR. It will boost or cut the on screen image exposure, regardless of the setting, to something that looks good. At the moment of exposure it does whatever you've set it to.
But, in dedicated video mode you need to set the type of exposure automation... A, M, P or S and also the ISO, auto or a specific setting. If the settings are wrong then it's just like looking through a reflex camera with the aperture stopped down.
Stick with automated video mode till you've got it sorted out.